Field of the Invention
The present invention relates to a device for measuring a power density distribution of a radiation source and to a method for measuring a power density distribution of a radiation source.
Description of the Background Art
Devices for measurement, including for online measurement and control of a laser beam, are known in a variety of embodiments. In this regard, DE 10 2007 062 825 A1 discloses a grating mirror, a monitoring device, a laser resonator and a beam guide which make it possible to couple out and focus a laser beam for online monitoring by means of a single optical element. This solution is has a grating mirror having a local grating period and a local alignment of the grating lines which are chosen in each case such that the grating mirror focuses a laser beam diffracted into a higher order of diffraction onto at least one focal point. In this case, the raw beam is directed via a mirror on which the grating is additionally applied. The complete beam diameter of the laser beam is directed onto the grating. By means of this grating, light is coupled out in only one order of diffraction, said light additionally being focused by the grating at a point, namely at the location of a detector.
The disadvantage of this arrangement is, in particular, that this focusing absolutely necessitates a locally changing grating constant over the area of the grating and the use of the raw beam. This also means that this solution is unsuitable with regard to a lateral intensity profile, that is to say a power density distribution, such as is required for example for industrial lasers for dividing wafers or generally in laser applications with short and high radiation intensities. A reflective element for short wavelengths is disadvantageous here. In DE 10 2007 062 825 A1, what is considered to be problematic about the use of (partly) transmissive materials such as ZnSe, GaAs, Ge, ZnS or Si as substrates for coupling out a portion of the laser beam is that the laser radiation coupled out passes through the substrate material, as a result of which cooling of the optical element can no longer be effected over the whole area from the rear side and therefore has to be effected along the circumference. Moreover, most of the materials mentioned above would have a poor thermal conductivity, which, together with the altered cooling, would result in a greater sensitivity toward destruction in the case of contamination of the optical element used for coupling-out. Furthermore, the focusing or imaging of the laser beam coupled out, if this is required, can generally be achieved only by means of a further optical element. However, this is not permanently critical in the case of the applications already mentioned above, since this is applicable in the IR range, in particular, and, moreover, measurements are not required continuously.
There are also known solutions in which the light distribution is either imaged onto a CCD camera via an optical construction or determined by means of absorbent diaphragms and the resulting intensity downstream of the diaphragm is recorded by means of a totally integrating detector (photodiode, calorimetric detector, etc.). In the case of measuring systems based on the diaphragm(s), in some instances different diaphragms are used, such as e.g. in the case of knife-edge (oblique and straight) and/or slits. From the data thus obtained, the profile is then subsequently calculated (deconvolution). These diaphragms are generally produced from metal or an absorbent metal layer on a transparent carrier substrate. The minimum aperture sizes are in the range of 2 to 5 μm.
This in turn has the disadvantage that, firstly, the CCD-based measuring systems withstand only low intensities. If the intensity becomes greater, then the laser beam has to be attenuated in order not to damage the CCD camera. This attenuation generally takes place by means of neutral density filters. However, distortions in the imaging of the laser beam can occur here on account of surface irregularities or the formation of thermal lenses. The measurement is thus corrupted greatly in some instances.
The diaphragm or knife-edge systems are therefore used at high intensities. However, here there is the disadvantage that, at very high intensities, in particular in the case of pulsed lasers, the absorbent diaphragm material is also eroded and the diaphragm thus degrades rapidly.